The present invention relates to fixing device for image fixing in an electrophotographic apparatus or the like.
More particularly, the present invention relates to a fixing device for an image forming apparatus of an electrophotographic type, wherein a transfer material on which toner is carried electrostatically and which toner comprises resin material, magnetic material, coloring material and the like, is passed through a nip. The nip is formed by heating means (roller, endless belt member or the like) and pressing means (roller, endless belt member or the like) which are press-contacted with each other and rotated, wherein the toner is subjected to heat and pressure during the passage of the transfer material through the nip to fuse and fix the toner on the transfer material.
In a conventional image forming apparatus, a toner image if formed by an image forming station, and the toner image is transferred onto a recording material such as a transfer sheet, recording sheet, photosensitive paper, electrostatic recording paper or the like, and thereafter, the toner image is fixed on a recording material into a permanent image by a heat fixing device of a heating roller type. Such a heating roller type apparatus comprises a fixing roller (fixing member) including a heater therein and a pressing roller press-contacted thereto to form a nip therebetween. The recording material is passed through the nip, by which the unfixed toner image formed and carried on the surface of the recording material is fixed on the surface of the recording material by the heat and pressure applied by the nip. For the heater, a halogen heater is used, wherein a radiant heat from the halogen heater is used, and therefore, the heat loss in the heat transfer from the halogen heater to the fixing roller is relatively large. In addition, since the fixing roller is heated indirectly from the halogen heater, the start-up time is relatively long. An IH type (electromagnetic induction heating type) heater is known with which energy consumption is reduced, and the start-up time is short. In the electromagnetic induction heating type, a high frequency current is applied to an excitation coil, and the high frequency magnetic field generated by the excitation coil is applied on the inner surface surface layer of the heat roller to generate eddy currents in the electroconductive layer of the fixing roller. By the eddy currents, self-heat-generation occurs in the heat roller due to the joule heat. Using such a heating method, the inner surface surface layer of the heat roller is the heat generating element, and therefore, a direct heating is effected. For this reason, the heat generating efficiency is high, and the heat roller can be quickly heated lip to a fixing temperature without difficulty. This accomplishes quick start-up. Also, since the efficiency is high, the electric energy consumption can be significantly reduced.
In order to properly fix the unfixed toner image on the recording material by heat in the fixing in device, a temperature control is effected to maintain a predetermined temperature of the fixing roller. As a temperature control system, it is considered that temperature of the fixing roller is detected by a temperature sensor, and in accordance with the result of the detection, the electric power supplied to the heater is linearly controlled (feed-back type). Referring first to FIG. 4, the description will be made as to the relation between the temperature and the electric power supplied to the induction coil corresponding to the temperature in the induction heating type apparatus. In FIG. 4, the ordinate represents an electric power value Po supplied to the heat source, and the abscissa represents the temperature T, that is, when the temperature of the fixing roller is Ta, the induction coil is supplied with the electric power Pa. By changing the supplied electric power to the induction coil in accordance with the temperature of the fixing roller, the temperature of the fixing roller can be maintained at the predetermined set temperature. Here, the temperature sensor for detecting the temperature of the fixing roller detects the temperature of the fixing roller at predetermined clearances (cyclic period of detection). Therefore, the temperature of the fixing roller is controlled to be constantly at the set temperature by determining the electric power supplied to the induction coil in accordance with the temperature detected at the sampling intervals. During the fixing operation, however, the amount of the toner carried on the transfer sheet is not constant, and more particularly, the amount is larger when the toner image on the transfer material is a color image than when it is a monochromatic image. Therefore, when the image is a color image, the heat quantity required to fix the image on the recording material is large, so that heat quantity removed from the fixing member is large. Then, the lowering degree of temperature in one detection interval or period of temperature detection of the temperature sensor is larger. As a result, the controlled temperature of the fixing member involves a larger ripple, so that non-uniformity in the glossiness may result in the fixed images. It is preferable that decrease of the temperature in one interval of temperature detection in the fixing operation is reduced by shortening the detection interval. In addition, the heat quantity required to fix the image is different depending on the kinds of the transfer sheets. For example, where the thickness of the recording material is large, the heat quantity removed from the fixing member during the fixing operation is large, and therefore, the lowering degree of the temperature in one interval is large. As a result, the ripple of the temperature of the fixing member is large with the result of non-uniformity in the glossiness in the image after the fixing. On the other hand, when the toner image is fixed on a thin recording material, the lowering degree of the temperature of the fixing member is relatively small as compared with the case of the thick recording material. Therefore, when the detection intervals are the same as with the thick recording material case, the electric power is supplied even when the temperature decrease is small, with the result of large ripple of the temperature of the fixing member. This may result in a non-uniformity in the glossiness of the fixed images.
Accordingly, it is a principal object of the present invention to provide a fixing apparatus or method wherein a temperature variation of the fixing member is minimized irrespective of fixing condition.
It is another object of the present invention to provide a fixing apparatus comprising a fixing member for fixing on a recording material an image formed on a recording material by heat; a temperature senior for sensing a temperature of said fixing member; detection interval changing means for changing an interval of detecting operation of said temperature sensor, wherein said detection interval changing means changes the interval in accordance with a thickness of the recording material.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.